I. Field of the Invention
The present invention relates generally to a spiral drum freezer apparatus rotatably mounted within an insulated enclosure for continuously freezing food articles. More particularly, the invention relates to a system directly measuring and indicating the extent of overdrive of a spiral belt frictionally engaging a periphery of the spiral drum.
II. Description of the Prior Art
Lotension spiral drive systems typically comprise an endless conveyor belt tightly arranged about a drum in an axially ascending or descending path. These spiral drive systems are typically used to route a large quantity of food packages through a freezer providing a chilling or even a cryogenic environment for rapidly freezing the food packages destined for use in other commercial markets, consumer markets, or both markets.
In a typical spiral drive system, an endless conveyor belt is guided in a helical path by support rails that progress either up or down the drum. The conveyor belt is disposed perpendicular to the drum surface and contacts the drum surface on an inner edge. The driving force on the conveyor belt results from the frictional engagement of the inner belt edge and the drum surface, similar in fashion to a rope in a capstan winch which is driven by a frictional force between the rope and the drum.
The amount of slippage between the belt and the drum is very important to the proper operation of a spiral conveyor. Too little slippage results in high conveyor belt tensions, while too much slippage results in high wear and pulsation phenomena in the conveyor belt. Therefore, an accurate determination and continual monitoring of the slippage parameter, called "overdrive", is very important.
In the past, the only way to determine the overdrive parameter was to measure it directly. For instance, one would stand at the outside conveyor belt edge of a running spiral and place an article on the conveyor belt. At that moment, the same radial spot on the drum surface is noted or marked. The drum is allowed to rotate one complete revolution, and when the spot marked on the drum surface passes the observer, the radial distance between this mark and the article riding on the belt is measured. Overdrive is defined as the distance that the drum leads the conveyor belt in one revolution of the drum, measured at the outside edge of the conveyor belt. This direct method of measurement has several disadvantages, including the possibility of inaccurate measurement in dark and cramped quarters, and the necessity of one being subject to the harsh processing atmosphere of a spiral conveyor, often exceeding -40.degree. F. Further, there was no way to continuously monitor overdrive, rather, overdrive was only sampled periodically.
U.S. Pat. No. 4,866,354 teaches a method of controlling the relative slippage or overdrive to set the overdrive at an acceptable value, but fails to teach a method of actually calculating overdrive other than by visually observing. Further, the speed of a gear motor driving the drum is obtained by means of a tachometer, which provides a voltage output to a summation circuit having a potentiometer in a feedback control circuit. Hence, the tachometer output is only indicative of motor speed which is not always indicative of the actual speed of drum rotation or overdrive. For instance, elements situated between the drive motor and the drum can fail, such as a safety clutch or a drive belt, producing conditions where the tachometer would not sense the true speed of a drum slowing down, having a disastrous effect on the wear of the spiral belt. Further, typical freezers see operating temperatures as low as -40.degree. F. or more and daily wash-downs with 160.degree. F. water containing detergents and sanitizers. The freeze/thaw cycle can cause condensation to form within parts of the tachometers such that pressure differentials can cause water to literally be sucked into any cracks of the speed measuring device. Therefore, tachometers are undesirable because they are unreliable in such a harsh environment.